1. Field of the Invention
The present invention relates to routing a received Signaling System 7 (SS7) message in a Common Channel Interoffice Signaling (CCIS) network of a out-of-band telecommunications system.
2. Description of the Related Art
Common Channel Interoffice Signaling (CCIS) networks provide out of band signaling for telecommunications networks such as public switched telephone networks. Most of the signaling communications for telephone networks utilize Signaling System 7 (SS7) protocol. An exemplary SS7 compliant CCIS network includes Service Switching Points (SSPs) (i.e., an SS7 capable telephony switch), Signaling Transfer Points (STPs), and data links between the STPs and SSPs and various telephone switching offices of the network.
As recognized in the art, the hardware and software operations of the SS7 protocol are divided into “layers”, similar to the Open Systems Interconnect (OSI) Network Model specified by the International Standards Organization (ISO). The “lowest levels” of the SS7 protocol include the Message Transfer Part (MTP) Level 1, Level 2, and Level 3. MTP Level 1 and Level 2 are equivalent to the OSI Physical Layer and the OSI Data Link layer, respectively. MTP Level 3, equivalent to the OSI Network Layer, provides message routing between signaling points in the SS7 network, and re-routes traffic away from failed links and signaling points and controls traffic when congestion occurs.
SS7 messages (also referred to as signal units) are routed throughout the SS7 network based on point codes specified within the SS7 message. In particular, each node of the signaling network is assigned a prescribed point code for purposes of addressing signaling messages throughout the SS7 network. The point code includes components that represent a network hierarchy based on the protocol being deployed.
One type of signal unit, known as a Message Signal Unit (MSU), includes a routing label which allows an originating signaling point to send information to a destination signaling point across the network. The routing label includes an originating point code (OPC) specifying the originating signaling node, a destination point code (DPC) specifying the destination for the SS7 messaging packet, and a signaling link selection (SLS) field. Hence, the selection of outgoing link is based on information in the DPC and SLS.
The size of the point code may vary depending on protocol; for example, each North American point code according to the American National Standards Institute (ANSI) uses 24 bits, whereas each point code specified by the International Telecommunication Union (ITU) uses 14 bits. In particular, an ANSI point code specifies a network hierarchy based on network, cluster, and member octets (e.g., 245-16-0 decimal). An octet is an 8-bit (i.e., 1-byte) value which can contain any value between zero and 255. Telcos with large networks have unique network identifiers while smaller operators are assigned a unique cluster number within networks 1 through 4 (e.g., 1-123-9). Network number 0 is not used; network number 255 is reserved for future use.
ITU-T point codes are pure binary numbers which may be stated in terms of zone, area/network, and signaling point identification numbers. For example, the point code 5557 (decimal) may be stated as 2-182-5 (binary 010 10110110 101).
The STPs are program controlled packet data switching systems. In operation, an STP will receive a packet data message from another node of the network, for example from an end office switching system. The STP analyzes the point code information in the packet and routes the packet according to a static routing table, also referred to as a translation table, stored within the STP. Any packet having a particular point code is output on a port going to the next CCIS signaling node specified by translation of that point code. Hence, the routing table stores for each (24-bit or 14-bit) point code a corresponding port address for outputting the packet to a specified link set.
FIG. 1 is a block diagram illustrating a public switched telephone network and the SS7 network that is used to control the signaling for the switched network. A switched telephone network has a common channel signaling network in the form of an SS7 network 12. The switched telephone network includes a series of central offices which are conventionally referred to as signaling points (SPs) in reference to the SS7 network. Certain of these SPs comprise end offices (EOs) illustrated at 14, 16, 18 and 20 as EOs 1–4 in FIG. 1. The EO may be a local or “end office” type switch.
Each signaling point has a point code comprising a 9-digit (24-bit) code assigned to every node in the SS7 network. For example, EO1 has a point code of 246-103-001, EO2 has a point code of 246-103-002, EO3 has a point code of 253-201-103, and EO4 has a point code of 253-201-104.
The end offices EO1 and EO2 represent end offices in the region of one regional operating company, while end offices EO3 and EO4 represent end offices of the region of a different operating company. The point code specifies a network ID, a number specifying a cluster, and a number specifying a member of the cluster (e.g., an SP). Specifically, each operating company has its own network ID, shown as 246 for Region 1 and 253 for Region 2. The number 103 in the designation 246-103-001, is the number of the cluster. An ANSI cluster can hold 255 SPs or members, the member being designated by the final 3 numbers. Thus 246 may represent C & P of Virginia Regional Operating Company, cluster 103, member EO2 for EO2 when viewed from an SS7 standpoint. The broken lines represent signaling links between the SPs; note that additional signaling links (not shown in FIG. 1) may be utilized for transporting signaling messages. Although not shown, the SPs are also connected by local trunks within a region and by inter-exchange carrier network (ICN) trunks across Access Tandems (ATs) 38 and 40 (AT1 and AT2). These SPs or ATs are shown as having point codes 246-103-003 and 253-201-101 respectively.
The SS7 network 12 comprises a series of Signal Transfer Points (STPs) shown at 41, 42, 44 and 46 designated STP1, STP2, STP3 and STP4, respectively. Each STP in a network is connected to the SPs in the network by A links indicated at 48, 50, 52 and 54. STP1 and STP2 constitute a mated pair of STPs connected by C links 56, while STP3 and STP4 constitute a mated pair connected by C links 58, each mated pair serving its respective transport area. It will be understood that there may be multiple mated pairs per region, one for each designated transport area. STP1 is connected to STP3 by B link 60 and to STP4 by D link 62. STP2 is connected to STP4 by B link 64 and to STP3 by D link 66.
As will be understood, the A, B, C and D links are physically identical with the designation relating to cost in terms of ease of access. The A links represent the lowest cost. B and D links have the same route cost with respect to SS7 so that the D designation is used only because it extends diagonally in the drawing. The C links are used to communicate between the two paired STPs for network management information and also constitute another route. The STPs in mated pairs have the same translations. Thus the translations in STP1 are the same as the translations in STP2, and the translations in STP3 are the same as the translations in STP4. The C links communicate between the paired STPs for network management information and SS7 message routing. The STP pair utilize the C links for redundancy to avoid loss of data. Therefore, unnecessary utilization of the C links causes congestion and interferes with the paired STPs efforts to back up data.
The SS7 network typically constitutes a highly redundant data network, generally a 56 K switched data circuit. By way of example, an SS7 message from EO2 to EO4 might travel any one of 8 possible routes. It could go from EO2 to STP1, from STP1 to STP3, STP3 to EO4. One variation on that route would be from STP 1 down the D link 62 to STP4 to EO4, and so forth. In the event that a link between STP3 and EO4 was lost, an SS7 route could be established from EO2 to EO4 via STP1 to STP3 and then via C link 58 to STP4 to EO4. However, that would be an undesirable route in unnecessarily using the C link. A links provide direct connectivity while C links provide circuitous routes using extra switches, a situation to be avoided. An alternate route would be from STP1 via D link 62 to STP4 to EO4. Another reason for not using the C link is to avoid tying up the entire STP3–STP4 pair.
As apparent from the foregoing, the static nature of the routing table in the corresponding signaling point (e.g., the STP) limits the flexibility of the packet switching system due to the difficulty associated with modifying the routing table. A modification to the table, for example to route certain signaling messages through alternate links because a portion of the network is disabled, requires a substitution of the entire table relating to messages intended for the now disabled portion of the network.
Moreover, the prescribed grouping of point code bits to prescribed network groups according to ANSI or ITU protocol imposes strict implementation-dependent limitations in provisioning the point codes to respective signaling nodes. Hence, network operators are required to manually provision point codes according to network topologies that may not suit their needs.
The ANSI standard allows for, and specifies in detail, a form of routing referred to as Cluster Routing. A “cluster route” is a route to a point code of which only the Network and Cluster portions have been specified, i.e., the first 16 bits. Hence, the cluster route enables one to specify a route to all members within that cluster. Such cluster routing, however, still may not be convenient for a telco attempting to provision a network.